ABSTRACT: Historically, under nutrition has been a major health concern. Recently however, over nutrition has also become a serious public health problem in the United States and other industrialized countries. With the new obesity epidemic we see increasing morbidity, mortality, and public health burden, particularly among reproductive-aged women. Whereas morbid obesity was once rare among pregnant women, a rapidly increasing proportion of obstetrics patients now have levels of obesity which dramatically increase their risk of serious pregnancy complications. These complications include increased risk of miscarriage, stillbirth, having a fetus that is too large leading to cesarean section or birth injuries for mom and baby from vaginal delivery, and death of the infant in the newborn period. More than 1/3 of women in the U.S. are now starting their pregnancies with a body mass index or 30 or greater, a condition that was unusual to rare 50 years ago. Given the serious consequences of added weight gain during pregnancy for obese women, such as gestational diabetes, pre-eclampsia, or cesarean delivery, the best strategy during their pregnancy may be to maintain a steady weight rather than gaining 15 pounds or more. Unfortunately, there is little research on the efficacy and feasibility of minimizing weight gain during pregnancy. This study is designed to address that problem. Two hundred women with BMIs of 30 or greater at the start of their pregnancy will be recruited for this feasibility test. All participants will be members of a non-profit managed care organization that provides high-quality obstetrics care. Patients who volunteer to participate will be randomly assigned to either a weight maintenance intervention or to usual care. Participants assigned to the intervention will participate in a weight maintenance program designed to help them eat a nutritionally balanced diet and to also control energy intake to minimize weight gain during their pregnancy. Participants (and their babies) in both groups will participate in follow- up assessments at 2 weeks postpartum, 6 months postpartum and one year postpartum. The primary outcome measures will be mothers' weight gain during pregnancy, the amount of weight retained after delivery, and the proportion of large for gestational age infants. Secondary outcomes will include multiple safety measures of the mothers and their babies. In addition to measures of safety, our secondary analyses will address the feasibility and acceptability of a weight management intervention among obese pregnant women.